1. Field of the Invention
The present invention relates to an improved, high rate method for combining waste materials to produce valuable products for agricultural, horticultural, sylvicultural or public use. Specifically, the present invention combines the steps of alkaline stabilization, beneficiating and pelletizing waste materials, such as wastewater treatment plant sludge or animal excrement and reaction with carbon dioxide, to thereby convert said waste materials to useful products such as soil conditioners or fertilizer supplements.
2. The Prior Art
Those skilled in sludge management practice are aware that methods for alkaline stabilization of dewatered sludge, beneficiating of waste materials and pelletizing of sludge are presently in use as separate and distinct processes. Environmental Protection Agency (EPA) publications that have reviewed stabilization and disinfection of municipal wastewater treatment plant sludges using alkaline materials conclude that alkaline stabilization has been demonstrated to effectively eliminate odors, improve bacterial and pathogenic organism control, provide stable material for application to agricultural land and provide alternate or supplemental treatment for existing facilities which are out of service due to overload, cleaning or repair.
In U.S. Pat. No. 4,306,978, dated Dec. 22, 1981, entitled "Method for Lime Stabilization of Wastewater Treatment Plant Sludges", granted to the applicant, the subject matter of which is incorporated herein by reference thereto, a method for lime stabilization of wastewater treatment plant sludge is disclosed, this method including the steps of dewatering sludge and rapidly and intimately mixing and reacting sludge cake with quicklime (calcium oxide) so as to produce stabilized sludge pellets.
The aforesaid patented method is advantageously and efficiently accomplished in a blender-dryer-reactor invented by the applicant and described in U.S. Pat. No. 3,941,357. This method has been widely accepted by the pollution control industry with many successful installations in the United States. Consulting Engineering firms have tabulated and reported data indicating the ability to alkaline stabilize dewatered sludge cake using the applicant's aforesaid method in as little time as ten seconds while still resulting in the complete destruction of pathogens. The performance was so outstanding as compared to other presently existing methods that plans and specifications prepared by the engineering consultants for a major city specified that applicant's method and apparatus be purchased on a "sole source" basis, that is, without competitive bidding even though it was a municipal project.
The new, high rate, improved method takes advantage of an embodiment of the applicant's present patented method in that the first step of the new method is to prepare a stabilized paste of the reacted materials, not pellets as taught by the present, patented method of the applicant. This initial step of preparing a pasty material is then further combined with additional, novel steps of beneficiating, pelletizing and high-rate curing of pellets to accomplish the new method.
In U.S. Pat. No. 4,270,279 dated June 2, 1981, Roediger also teaches a method for alkaline stabilization of dewatered sludge cake which results in the formation of pellets. The object is that an inexpensive method is provided for sterilizing dewatered sludge cake which is discharged from belt presses wherein the sludge cake is broken into ball-like particles having diameters ranging from 1 to 10 millimeters, and coating these particles with quicklime (calcium oxide). The quicklime reacts exothermically with the surfaces of said ball-like particles resulting in a product which can be used as an agricultural fertilizer. The means for mixing and reacting the sludge with quicklime includes a paddle blender or pug mill.
Also, the beneficiating of wastewater treatment plant sludge has been well known by those skilled in sludge management. As reported in Compost Science Magazine, July/August 1975 issue, Messrs Baker, Welch, Stout and Doty of Pennsylvania State University concluded, as a result of test work, that a waste material, cement kiln dust, could advantageously be utilized in place of other alkaline materials for processing sewage sludge. And, they reported that beneficiating of the dewatered filter cake also took place when cement kiln dust was added. This is due to the nutrients, such as potassium and magnesium, which are contained in cement kiln dust, combining with the valuable nitrogen, phosphorus, trace minerals and organic matter of sludge so as to be advantageous for crop production when applied to land.
U.S. Pat. No. 4,554,002 granted to Nicholson discloses a method for beneficiating "low percentage solids" waste water treatment sludge, without prior dewatering, by mixing kiln dust containing a percentage of calcium oxide, without compacting said mixture, and aging for sufficient period of time to produce a product which can be applied to land as well as use as a soil conditioner and fertilizer supplement.
The aforesaid separate process methods are presently in practice today, but all have limitations. Because of the increasing demand for viable alternatives for disposing of wastewater treatment plant sludges and the promotion of practices that provide for the beneficial use of sludge, the new method of the invention addresses the deficiencies of the prior art methods and combines the processes with a novel high-rate curing step to provide a valuable product from the aforesaid waste materials.
A limitation of all prior art methods is the inability to rapidly process a uniform, beneficiated product that can reduce or eliminate the relatively long storage and curing periods to thereby prepare a product for immediate beneficial uses. None of the prior art methods provide a product with an initially high green strength. High green strength is defined as the physical condition of the pellets or product which can withstand sufficient compressive and shear forces so as not to fracture or break down when handled with mechanical equipment, such as a mechanical spreader used for dispersing material for land application. The inability of prior art methods to form uniform pellets with high green strength is due to many factors, such as continuous variations of the sludge; variations of the additive waste material components to the mixture; variable calcium oxide content of the waste materials such as kiln dusts; the retention time that the material is acted upon in the mixer-reactor with resultant variations in shear forces; speed changes for increased or decreased flow rates also producing variable shear forces and changes of viscosity of the processed material. Most plant operators cannot respond to these many variations which, in effect, has prevented the combining of the steps of stabilizing, beneficiating and pelletizing of the waste materials so as to result in a product of uniform particle size, beneficiated, with high green strength, and yet still efficiently produced and cost effective.
As an example of the limitations of the prior art methods, Water/Engineering & Management Magazine, December, 1988 issue, reported that the beneficiating method of U.S. Pat. No. 4,554,002 required approximately three days for disinfection and approximately seven days to produce a friable stage of the resultant product so that the product could be granulated. In U.S. Pat. No. 4,270,279, the Roediger method specifies time for completion of the process and specifies an additional process step of storing the product for about four additional hours so as to increase the pellet green strength. Additionally, the pellet size as taught by Roediger is 1 to 10 millimeters in diameter which, relatively speaking, is nonuniform as compared to the pellet size of the method of the invention.
Each of the aforesaid stabilization methods have many individual limitations and disadvantages when compared with the method according to the invention as hereinafter described.
A severe limitation of the prior art methods is the need to store the resultant sludge mixtures for relatively long periods of time to provide sufficient curing to enable material handling when applying the products of the methods to land by means of conventional equipment. This requirement to store material therefor requires costly land storage area as well as additional labor and material handling equipment to transport material to and from the storage area. This storage period is mandatory for proper preparation of the reacted materials when using prior art methods. It is believed that an additional chemical reaction takes place during the storage and curing period. The chemical reaction is a relatively slow reaction between calcium hydroxide contained in the stabilized sludge and the small amount of carbon dioxide in the atmosphere. This reaction will thereby form calcium carbonate.
For a better understanding, a brief review of the chemical reactions involved follows: First, for stabilization and disinfection of sludge, an alkaline material such as calcium oxide (quicklime) or waste materials containing calcium oxide, such as kiln dusts, are added to dewatered sludge. The calcium oxide reacts exothermically with the free water of each sludge particle and forms calcium hydroxide plus heat.
As an alternate alkaline material, calcium hydroxide, commonly called hydrated lime, can be used for stabilization in lieu of calcium oxide. In either case, the resultant sludge mixture contains calcium hydroxide. During storage, or enhanced by mechanical aeration, the calcium hydroxide will react slowly with carbon dioxide in the atmosphere to form calcium carbonate.
It is well known however, that the atmosphere only contains 0.035% carbon dioxide. Therefore, for example, the Nicholson method requires up to seven days of curing and storage to prepare the product for use, this actual requirement due to the limited availability of carbon dioxide. This extremely slow prior art reaction rate can be compared with the new, high-rate method according to the invention wherein the reaction and formation of calcium carbonate is immediate by means of the use of 100% carbon dioxide or up to 100% carbon dioxide. This is contrasted with relying on the extremely low percentage of carbon dioxide in the atmosphere as in prior art methods. Thus, the need to store and cure reacted materials prior to use is eliminated, thereby overcoming a major limitation of prior art methods.